


self indulgent nesting bullshit

by avosettas



Category: Undertale (Video Game)
Genre: Alternate Universe - Swapfell (Undertale), F/M, Idiots in Love, Nesting, Original Monster Species, Post-Undertale Pacifist Route, Swapfell Sans (Undertale), Weird Biology, read notes please, uhhhhhh
Language: English
Status: Completed
Published: 2020-10-03
Updated: 2020-10-03
Packaged: 2021-03-07 23:01:26
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 1,516
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/26785573
Author URL: https://archiveofourown.org/users/avosettas/pseuds/avosettas
Summary: “OH GOD, DOES THIS HAVE TO DO WITH THE BLEEDING THING?”~The bird hormones and the human hormones, they aren't lining up.
Relationships: Sans (Undertale)/Reader
Kudos: 72





	self indulgent nesting bullshit

**Author's Note:**

> i never thought id post this lmao bu t here we are!
> 
> anyway. despite this being very much self inserty it works as a reader insert i think? reader is described as afab and having barn owl wings. but you could substitute barn owl for pretty much any kind of bird, if you want. 
> 
> basically i wanted to write about siren (my version of sirens) reproduction using ym self insert who is Me but With Wings nd Magic.

It’s strange that you aren’t awake when Mal returns from his morning run. The only sound as he walks in is the jingle of the cats running to greet him (and beg for their breakfasts, tiny beasts…). It’s still early, though. Plenty of time to get up and moving.

Your alarm is still blaring when he walks back into the bedroom. He shuts it off himself, and then jostles you as gently as he can. 

“‘s it morning?” It’s more of a grunt than actual language, and he chuckles. 

“LOOK AT THE BRAINS ON YOU, DEAREST,” he teases. Normally, even so early in the morning, you would giggle at that and fire something back, but today you just burrow back into the blanket and lean into his hand as it smooths back your hair. 

“Don’t feel great,” you grumble. 

“YOU DO FEEL FEVERISH,” Mal observes. “...MAYBE YOU SHOULD STAY HOME TODAY?” 

“What will Edge do without my needling?” Your joke comes out like a whine, and he sighs, hiking the covers up a bit more. 

“HE’LL LIVE. I’LL FEED YOUR MONSTROSITIES BEFORE I LEAVE.” 

“Thank you.” You already look like you’re falling back asleep. “Love you.” 

“I LOVE YOU TOO, DEAR. MAKE SURE YOU TAKE YOUR MEDICATION.” He rattles the pillbox on the nightstand for effect, and you nod slowly.

~

“heya, mal.” 

“OH DEAR GOD, NOT YOU.” Sometimes, Mal wondered how he and some of his doubles were the same people - Blue, he could see. Barely. Red and Sans and Slate, not so much, though he supposed it was expected since he was from a “Swap” Underground. 

“aw, definitely not the warm welcome i egg-spected,” Sans says, falling into step besides Mal. 

“YOU DON’T EVEN WORK HERE,” Mal grumbles. “AND WHAT’S WITH THE EGG PUN?”

“well, a little birdie told me she wasn’t feeling great,” Sans explains, leaning on the wall besides the vending machine as Mal stops to buy chisps. “and, uh, she let slip that she explained the human hormones to you, but not the bird ones.” 

“OH GOD, DOES THIS HAVE TO DO WITH THE BLEEDING THING?” 

“heh, that’s the same response pap had when she complained about it to him and ended up having to explain her whole reproductive cycle to us…” Sans straightens a bit, and then holds out a book. “tangentially, it’s related, i guess. here. she left it with her stuff when she moved out; guess she forgot it or didn’t need it anymore.” 

It’s a book titled _A-Z Monster Biology: P through T_. 

“AN ENCYCLOPEDIA?” 

“well, she’s not gonna be explaining it to you herself for a bit, so… yeah, an encyclopedia.” Sans winks. “see you later.” And he disappears, leaving Mal to balance his packet of chisps and the stupid encyclopedia. 

~

 _As Sirens are considered both mammalian and avian, the bodies of females show reproductive tendencies of both the class_ Aves _and the class_ Mammalia _, including but not limited to milk production, mammary glands, egg laying, and menstrual cycles…_

_A Siren’s menstrual cycle will not line up with her nesting time. It is entirely possible for it to happen, but not the norm. Menstruation occurs once a month, while nesting will occur about two to four times a year, three to six months apart. During nesting, the Siren will lay a single sterile egg matching the species of bird with which she shares her wings…_

_The most noticeable sign of nesting is a high fever and sleepiness a few hours prior to the hormonal-magic flood of the system. Occasionally, there will be a small-scale molt. Following this, the female will create a nest, generally only big enough to fit herself and her eggs. She will remain in this nest until the hormones wear off, generally about three days later…_

_Sirens are generally solitary creatures, laying their eggs and coaxing their young from the shells alone. However, the mate may be allowed in the nest…_

_It is not uncommon for the Siren to become nonverbal, communicating in Hands, or simply trilling, chirping, hissing, etc. She may also become overstimulated, due to her enhanced senses…_

Mal has no appetite for his chisps, now. Bodily functions, no matter how normal, are not his strong suit. 

He keeps reading, as the writer goes into how sirens are humanoid, but… not. 

_Sirens will inherit the features of their subspecies; an owl Siren, for instance, will be a silent flyer. This determines what sort of eggs they will lay, the pattern of their feathers, their molting patterns, etc.._

_All Sirens have sharp teeth, claws, and wings. Their senses are generally the same as a humans, although certain species excel in hearing and/or sight. All Sirens have a keen sense of the SOUL and magic, more so than other monsters..._

So his datemate is… about to nest. Mal sighs and puts his head into his hands. You’ll be out of commission and possibly nonverbal for up to three days. 

He’ll manage. 

~

He would guess you haven’t been out of bed, but the cat bowls have been replaced in their spot in the cabinet, and the spare blanket on the couch is missing. 

Mal makes his way back to the bedroom, not exactly sure what he should be expecting. But it’s just you. 

You’ve taken not only the comforter, but also what seems to be all the spare blankets, and wrapped them into a crude circle. There’s a single, small egg in the center with you, and a few feathers scattered around the blankets.

“LIKE THOSE BIRDS YOU WERE TELLING ME ABOUT, I SUPPOSE,” Mal muses, settling himself next to the rim of the nest. “THE ONES THAT LAY STERILE EGGS AND THEN GET UPSET IF YOU TAKE THEM AWAY TOO SOON.”

Your eyes are closed, but you hum a bit in agreement. Then, with a long little trill, you pull him down into the nest. It’s not exactly big enough, but it’s comfy. 

‘I’M NOT USED TO HEARING YOU MAKE BIRD NOISES THAT AREN’T… SCREAMING.”

“Barn owl,” you reply softly, before chirping and then sort of… headbutt his chest. 

“YES, OF COURSE. I’M SUPPOSED TO KNOW WHAT THAT MEANS, HM?” It gets a giggle out of you. “SANS GAVE ME YOUR ENCYCLOPEDIA, BY THE WAY.”

You nod this time, still holding him close. 

“...I DO HAVE TO MAKE US DINNER, DEAR.” 

You whine a little, but don’t protest aside from that, retreating back into the blankets and curling around the egg. It’s small, maybe a little bigger than your average chicken’s egg, and an off-white shade. Mal supposes that’s what Barn owl eggs look like.

He makes toast for you and a microwave burrito for himself, since the encyclopedia said that lighter food was better right now. And also, you like toast. (And eggs, but he’d argued with himself over the stove for five minutes before deciding that probably wasn’t appropriate). 

He watches you eat, chewing slowly on the toast (which was probably crisper than you’d prefer, but oh well). You stare at him throughout the rest of the informal little meal - it reminds him of a cat waiting to pounce. 

And pounce you do. Mal puts his burrito down on the plate for a moment to grab one of the water bottles he’d brought into the room, and when he turns back around you’re picking bits of chicken out of his dinner. 

“EXCUSE ME.” 

You trill at him. It sounds kind of smug. 

~

It goes like that for _five days_ , not the three the book had promised. 

Mal sleeps on the couch, because your nest is too small for him. He’s checked on you in the middle of the night - you sleep curled up tightly in it, like one of your cats, or a pretzel. If you were verbal he’s sure you’d be complaining about your back. 

Day six comes around and he leaves for his morning run. He folds the (very few) blankets he’s been using on the couch before he leaves. 

When he comes back, they’re gone. The cats don’t greet him; they seem to be busy at their food dishes already. 

“DEAREST…?” He doesn’t hear anything back - maybe you fell back asleep. When he makes it to the bedroom though, it’s more obvious that your Barn owl hearing has simply lost to your audio processing issues again. 

You’re dressed, finally, suitable enough for work at the Embassy, and you’ve made the bed. Or, rather, you’re finishing making the bed, trying to get some final blankets up into the closet. 

Considering the angle, and the fact that you have to reach up to that high shelf where the spare blankets usually go… Mal ogles. 

And you, ever the perceptive one, notice. “Are you going to stare at my butt, or are you going to help me with this?” 

“WELL, I AM ENJOYING THE VIEW…”

“Guess that’s what I get for not telling you about my nesting cycle beforehand,” you huff, tossing the blankets messily into the top of the closet. “I’m sure Sans was less than helpful. I should have made Paps tell you.” 

“EH, THE BOOK WAS HELPFUL ENOUGH.”


End file.
